The invention relates to a multistage process for continuous and phosgene-free preparation of cycloaliphatic diisocyanates, diisocyanates produced by the process and use of the diisocyanates for forming polymers or coatings.
1. Field of the Invention
The synthesis of isocyanates may be via a series of different routes. The phosgene route is used for the industrial scale preparation of isocyanates and is the oldest and still predominates today. This process is based on the reaction of amines with phosgene. A disadvantage of the phosgene process is that phosgene must be used which, as a consequence of its toxicity and corrosivity, places particularly high safety and equipment requirements on its handling on an industrial scale.
2. Description of the Related Art
There are several processes for preparing isocyanates which avoid the use of phosgene on an industrial scale. The term phosgene-free process is frequently used in connection with the conversion of amines to isocyanates using alternative carbonylating agents such as, for example, urea or dialkyl carbonate (U.S. Pat. Nos. 4,713,476; 5,087,739; 4,268,683; 6,204,409).
The urea route is based on the urea-mediated conversion of diamines to diisocyanates via a two-stage process. In a first step, a diamine is reacted with alcohol in the presence of urea or urea equivalents (for example alkyl carbonates, alkyl carbamates) to give a diurethane which typically passes through an intermediate purification stage and is then thermally cleaved in a second step to form a diisocyanate and alcohol (U.S. Pat. Nos. 5,087,739; 4,713,476; 5,386,053). Alternatively, the actual urethane formation may also be preceded by the separate preparation of a diurea by selectively reacting the diamine with urea (U.S. Pat. No. 5,360,931). Also conceivable is a two-stage sequence consisting of a partial reaction of urea with alcohol in the first step and subsequent metering in and urethanization of the diamine in the second step (U.S. Pat. No. 5,744,633).
The thermal cleavage of urethanes to the corresponding isocyanates and alcohols has been known for some time and can be carried out either in the gas phase at high temperatures or at relatively low temperatures in the liquid phase. However, a problem with both procedures is that the thermal stress inevitably also causes undesired side reactions to take place which reduce the yield and lead to the formation of resinifying by-products which considerably disrupt the course of an industrial process as a result of deposits and blockages which may form in reactors and workup apparatus.
There has been no shortage of suggestions of chemical and process technology measures to achieve yield improvements and limit the undesired by-product formation. For instance, a series of documents describes the use of catalysts which accelerate the cleavage reaction of the urethanes (DE 10 22 222, U.S. Pat. Nos. 3,919,279, 4,081,472). Indeed, it is entirely possible in the presence of suitable catalysts, of which a multitude of basic, acidic and also organometallic compounds are known, to increase the isocyanate yield in comparison to the uncatalyzed variant. However, the formation of undesired by-products can not be prevented by the presence of a catalyst. The same applies to the additional use of inert solvents in order to ensure uniform distribution of the heat supplied and of the catalyst in the reaction medium, as recommended in U.S. Pat Nos. 3,919,279 and 4,081,472. However, the process which utilizes solvents boiling under reflux results in a reduction in the space-time yield of isocyanates and is additionally hindered with the disadvantage of additional high energy demands.
Examples for thermally catalyzed cleavage of monourethanes disclose the partial discharge of the reaction mixture to remove resinifying by-products formed in the course of the urethane cleavage are described in U.S. Pat. No. 4,386,033. This procedure serves to prevent deposits and blockages in reactors and workup units. There is however no disclosure which points to a yield-increasing utilization of the partial discharge. U.S. Pat. No. 4,388,246 describes a similar solution in which thermolysis is carried out in the presence of solvents whose purpose is apparently to better absorb the non-volatile by-products. Here also, the partial discharge is not utilized for the purposes of yield optimization.
U.S. Pat. No. 5,087,739 discloses that a yield increase can be achieved when higher molecular weight by-products which are formed in the cleavage reactor during the cleavage of diurethanes and may or may not be utilized to ensure a disruption-free and selective reaction, are discharged substantially continuously out of the reactor and subsequently converted for the most part in the presence of alcohol and then recycled into the diurethane preparation. The procedure is associated with high energy demands since nonutilizable by-products are removed from the effluent of the diurethane preparation by distillation, and all of the diurethane has to be evaporated. In contrast to U.S. Pat. No. 5,087,739, the urethanization effluent in the process of U.S. Pat. No. 5,386,053 is divided into two substreams of which only one is freed by distillation of its high-boiling, nonutilizable by-products, before the combined diurethane streams are fed to the deblocking reaction in the cleavage reactor. In addition, the continuous cleavage reactor discharge in U.S. Pat. No. 5,386,053 is recycled directly, i.e., without a reurethanization step, into the diurethane synthesis.
The method of U.S. Pat. No. 5,386,053 has the consequence that some of the high boiler components from the diurethane synthesis, via the deblocking or cleavage stage, get back into the diurethane preparation and into the diurethane purification procedure.